I've only ever used the one that I bought and don't know anyone else with one.   
I'm stocked with the job it does in such a quick time, it still spins me out.  I imagine most of them would be pretty similar in result.  I believe the higher the frequency, the better it cleans but couldn't confirm that. 

I chose the one I got based on size and the inbuild heater.

I bought this one.....
http://www.ebay.com.au/itm/2500ml-Ultrasonic-Cleaner-Tank-Bath-with-Bracket-Jewellery-Ring-Dentures-/181066864127?pt=LH_DefaultDomain_15&hash=item2a286d15ff